commitlog_replayer: Fix calculation of global min pos per shard
commitlog_replayer: Fix calculation of global min pos per shard

If a CF does not have any sstables at all, we should treat it
as having a replay position of zero. However, since we also
must deal with potential re-sharding, we cannot just set
shard->uuid->zero initially, because we don't know what shards
existed.

Go through all CF:s post map-reduce, and for every shard where
a CF does not have an RP-mapping (no sstables found), set the
global min pos (for shard) to zero.

Fixes #1372

Message-Id: <778e2855f4310635f4ad5934e382812a0d20fd10@scylladb.com>

kjniemi/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,kjniemi/scylla,kjniemi/scylla,raphaelsc/scylla,duarten/scylla,avikivity/scylla,raphaelsc/scylla